Letters to the editor, Nov. 15, 2017

Using the results of the Mexico study published earlier this fall to justify calling the community fluoridation of water an "experiment" is hardly fair. The longitudinal study of children born to low-to-moderate income level mothers, recruited through hospitals in Mexico City does provide individual level data for both exposure and outcomes, which is a strength of the research. The methodology used to measure both fluoride exposure as well as cognitive performance was well-established. However, the research suffers from a number of limitations.

There are problems with potential bias in the way the mothers were recruited, as well as a lack of information on other important details, such as the other sources of fluoride (Mexico does not add fluoride to drinking water but instead, salt is fluoridated to 250 parts per million) and the possible confounding impact of lead, or other environmental exposures such as arsenic or iodine. Although prenatal testing was targeted for each trimester, many of the children included in the analysis had only one maternal urinary result, making it impossible to assess true exposures. In addition, there were differences noted between the two groups of women studied. One group had higher bone levels of lead and it appears that participants who had cognitive measurements had higher prenatal levels of mercury than those who were excluded due to missing data.

The authors conclude with "Our findings must be confirmed in other study population, and additional research is needed to determine how the urine fluoride concentrations measured in our study populations are related to fluoride exposures resulting from both intentional supplementation and environmental contamination."

It is a stretch to advocate for anything, based on the results of the Mexican research. Coincidentally, at the same time that the Mexico paper was published, another one published in the Canadian Journal of Public Health found no relationship between measures of urinary fluoride and the diagnosis of a learning disability in Canadian children aged 3 to 12 years of age. Readers should find that very reassuring.